Replace Filename before underscore_ using powershell

As per my comment, this is literally one line of code to accomplish this task.

You must submit code (show your effort) for folks to want to help you, but I am putting this here as a launch point to get you in the right direction. Many Youtube videos show you how to do this and more. Search Youtube for 'Beginning PowerShell' or 'PowerShell file and folder management'.

Basic/simple string replacement:

# Replace the start of a string up to and including the underscore. https://regex101.com
'ABC_123.xls', 'CAB_234.xls' -replace '.*_'
# Results
<#
123.xls
234.xls
#>

Create test files:

'ABC_123.xls', 'CAB_234.xls' | 
ForEach-Object {New-Item -Path 'D:\Temp' -Name $PSItem -ItemType File}
# Results
<#
    Directory: D:\Temp


Mode                 LastWriteTime         Length Name
----                 -------------         ------ ----
-a----         11-Oct-22     18:14              0 ABC_123.xls
-a----         11-Oct-22     18:14              0 CAB_234.xls
#>

Get a list of files by full name and change it:

(Get-ChildItem -Path 'D:\Temp' -Filter '*.xls').FullName | 
ForEach-Object { Rename-Item -Path $PSItem -NewName ($PSItem -replace '.*_') -WhatIf}
# Results
<#
What if: Performing the operation "Rename File" on target "Item: D:\Temp\ABC_123.xls Destination: D:\Temp\123.xls".
What if: Performing the operation "Rename File" on target "Item: D:\Temp\CAB_234.xls Destination: D:\Temp\234.xls".
#>

Remove the WhatIf to make it real.